I will probe the properties and evolution of the genetic network that underlies circadian rhythm in the cyanobacteria Synechococcus elongatus PCC 7942 to better understand the molecular mechanism of circadian regulation, its plasticity (evolvability) and general properties of gene networks. To do this, I propose three sets of experiments. (1) Systematically and specifically vary parameters within the circadian genetic network in S. Elongatus (i.e. abundance, stability and biochemical properties of clock genes) and quantify differences in circadian rhythms. This will provide a bottom-up approach to determine the relative contribution of these parameters to phenotype, the robustness of the network to perturbations in these parameters, and the range of phenotypes that the system can be pushed to have with relatively simple changes. (2) Determine the circadian period of thousands of single and double gene knockouts to determine the number of genes that influence circadian period and to what extent. (3) Experimentally evolve S. elongatus to have altered circadian rhythms by selection in different light-dark cycles to directly probe the evolvability of the circadian genetic network. Insights here will be useful in our understanding of circadian control and its pathologies in higher organisms, as well as our understanding of other genetic networks.